Network Working Group D. Thaler
Internet-Draft Microsoft
Intended status: Informational October 31, 2016
Expires: May 4, 2017
Privacy Considerations for IPv6 Adaptation Layer Mechanismsdraft-ietf-6lo-privacy-considerations-04
Abstract
This document discusses how a number of privacy threats apply to
technologies designed for IPv6 over various link layer protocols, and
provides advice to protocol designers on how to address such threats
in adaptation layer specifications for IPv6 over such links.
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to the same privacy threats from off-link attackers, since experience
shows they are often leaked by upper-layer protocols such as SMTP,
SIP, or DNS.
For these reasons, [I-D.ietf-6man-default-iids] recommends using an
address generation scheme in [RFC7217], rather than addresses
generated from a fixed link-layer address.
Furthermore, to mitigate the threat of correlation of activities over
time on long-lived links, [RFC4941] specifies the notion of a
"temporary" address to be used for transport sessions (typically
locally-initiated outbound traffic to the Internet) that should not
be linkable to a more permanent identifier such as a DNS name, user
name, or fixed link-layer address. Indeed, the default address
selection rules [RFC6724] now prefer temporary addresses by default
for outgoing connections. If a device needs to simultaneously
support unlinkable traffic as well as traffic that is linkable to
such a stable identifier, this necessitates supporting simultaneous
use of multiple addresses per device.
2. Amount of Entropy Needed in Global Addresses
In terms of privacy threats discussed in [RFC7721], the one with the
need for the most entropy is address scans of routable addresses. To
mitigate address scans, one needs enough entropy to make the
probability of a successful address probe be negligible. Typically
this is measured in the length of time it would take to have a 50%
probability of getting at least one hit. Address scans often rely on
sending a packet such as a TCP SYN or ICMP Echo Request, and
determining whether the reply is an ICMP unreachable error (if no
host exists with that address) or a TCP response or ICMP Echo Reply
(if a host exists), or neither in which case nothing is known for
certain.
Many privacy-sensitive devices support a "stealth mode" as discussed
in Section 5 of [RFC7288] or are behind a network firewall that will
drop unsolicited inbound traffic (e.g., TCP SYNs, ICMP Echo Requests,
etc.) and thus no TCP RST or ICMP Echo Reply will be sent. In such
cases, and when the device does not listen on a well-known TCP or UDP
port known to the scanner, the effectiveness of an address scan is
limited by the ability to get ICMP unreachable errors, since the
attacker can only infer the presence of a host based on the absense
of an ICMP unreachable error.
Generation of ICMP unreachable errors is typically rate limited to 2
per second (the default in routers such as Cisco routers running IOS
12.0 or later). Such a rate results in taking about a year to
completely scan 26 bits of space.
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The actual math is as follows. Let 2^N be the number of devices on
the subnet. Let 2^M be the size of the space to scan (i.e., M bits
of entropy). Let S be the number of scan attempts. The formula for
a 50% chance of getting at least one hit in S attempts is: P(at least
one success) = 1 - (1 - 2^N/2^M)^S = 1/2. Assuming 2^M >> S, this
simplifies to: S * 2^N/2^M = 1/2, giving S = 2^(M-N-1), or M = N + 1
+ log_2(S). Using a scan rate of 2 per second, this results in the
following rule of thumb:
Bits of entropy needed = log_2(# devices per link) + log_2(seconds
of link lifetime) + 2
For example, for a network with at most 2^16 devices on the same
long-lived link, and the average lifetime of a link being 8 years
(2^28 seconds) or less, this results in a need for at least 46 bits
of entropy (16+28+2) so that an address scan would need to be
sustained for longer than the lifetime of the link to have a 50%
chance of getting a hit.
Although 46 bits of entropy may be enough to provide privacy in such
cases, 59 or more bits of entropy would be needed if addresses are
used to provide security against attacks such as spoofing, as CGAs
[RFC3972] and HBAs [RFC5535] do, since attacks are not limited by
ICMP rate limiting but by the processing power of the attacker. See
those RFCs for more discussion.
If, on the other hand, the devices being scanned for respond to
unsolicited inbound packets, then the address scan is not limited by
the ICMP unreachable rate limit in routers, since an adversary can
determine the presence of a host without them. In such cases, more
bits of entropy would be needed to provide the same level of
protection.
3. Potential Approaches
The table below shows the number of bits of entropy currently
available in various technologies:
+---------------+--------------------------+--------------------+
| Technology | Reference | Bits of Entropy |
+---------------+--------------------------+--------------------+
| 802.15.4 | [RFC4944] | 16+ or any EUI-64 |
| Bluetooth LE | [RFC7668] | 48 |
| DECT ULE | [I-D.ietf-6lo-dect-ule] | 40 or any EUI-48 |
| MS/TP | [I-D.ietf-6lo-6lobac] | 7 or 64 |
| ITU-T G.9959 | [RFC7428] | 8 |
| NFC | [I-D.ietf-6lo-nfc] | 5 |
+---------------+--------------------------+--------------------+
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Such technologies generally support either IEEE identifiers or so
called "Short Addresses", or both, as link layer addresses. We
discuss each in turn.
3.1. IEEE-Identifier-Based Addresses
Some technologies allow the use of IEEE EUI-48 or EUI-64 identifiers,
or allow using an arbitrary 64-bit identifier. Using such an
identifier to construct IPv6 addresses makes it easy to use the
normal LOWPAN_IPHC encoding with stateless compression, allowing such
IPv6 addresses to be fully elided in common cases.
Global addresses with interface identifiers formed from IEEE
identifiers can have insufficient entropy to mitigate address scans
unless the IEEE identifier itself has sufficient entropy, and enough
bits of entropy are carried over into the IPv6 address to
sufficiently mitigate the threats. Privacy threats other than
"Correlation over time" can be mitigated using per-network randomized
link-layer addresses with enough entropy compared to the link
lifetime. A number of such proposals can be found at
<https://mentor.ieee.org/privecsg/documents>, and Section 10.8 of
[BTCorev4.1] specifies one for Bluetooth. Using routable IPv6
addresses derived from such link-layer addresses would be roughly
equivalent to those specified in [RFC7217].
Correlation over time (for all addresses, not just routable
addresses) can be mitigated if the link-layer address itself changes
often enough, such as each time the link is established, if the link
lifetime is short. For further discussion, see
[I-D.huitema-6man-random-addresses].
Another potential concern is that of efficiency, such as avoiding
Duplicate Address Detection (DAD) all together when IPv6 addresses
are IEEE-identifier-based. Appendix A of [RFC4429] provides an
analysis of address collision probability based on the number of bits
of entropy. A simple web search on "duplicate MAC addresses" will
show that collisions do happen with MAC addresses, and thus based on
the analysis in [RFC4429], using sufficient bits of entropy in random
addresses can provide greater protection against collision than using
MAC addresses.
3.2. Short Addresses
A routable IPv6 address with an interface identifier formed from the
combination of a "Short Address" and a set of well-known constant
bits (such as padding with 0's) lacks sufficient entropy to mitigate
address scanning unless the link lifetime is extremely short.
Furthermore, an adversary could also use statistical methods to
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determine the size of the L2 address space and thereby make some
inference regarding the underlying technology on a given link, and
target further attacks accordingly.
When Short Addresses are desired on links that are not guaranteed to
have a short enough lifetime, the mechanism for constructing an IPv6
interface identifier from a Short Address could be designed to
sufficiently mitigate the problem. For example, if all nodes on a
given L2 network have a shared secret (such as the key needed to get
on the layer-2 network), the 64-bit IID might be generated using a
one-way hash that includes (at least) the shared secret together with
the Short Address. The use of such a hash would result in the IIDs
being spread out among the full range of IID address space, thus
mitigating address scans, while still allowing full stateless
compression/elision.
For long-lived links, "temporary" addresses might even be generated
in the same way by (for example) also including in the hash the
Version Number from the Authoritative Border Router Option
(Section 4.3 of [RFC6775]), if any. This would allow changing
temporary addresses whenever the Version Number is changed, even if
the set of prefix or context information is unchanged.
In summary, any specification using Short Addresses should carefully
construct an IID generation mechanism so as to provide sufficient
entropy compared to the link lifetime.
4. Recommendations
The following are recommended for adaptation layer specifications:
o Security (privacy) sections should say how address scans are
mitigated. An address scan might be mitigated by having a link
always be short-lived, or might be mitigated by having a large
number of bits of entropy in routable addresses, or some
combination. Thus, a specification should explain what the
maximum lifetime of a link is in practice, and show how the number
of bits of entropy is sufficient given that lifetime.
o Technologies should define a way to include sufficient bits of
entropy in the IPv6 interface identifier, based on the maximum
link lifetime. Specifying that randomized link-layer addresses
can be used is one easy way to do so, for technologies that
support such identifiers.
o Specifications should not simply construct an IPv6 interface
identifier by padding a short address with a set of other well-
known constant bits, unless the link lifetime is guaranteed to be
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extremely short or the short address is allocated by the network
(rather than being constant in the node). This also applies to
link-local addresses if the same short address is used independent
of network and is unique enough to allow location tracking.
o Specifications should make sure that an IPv6 address can change
over long periods of time. For example, the interface identifier
might change each time a device connects to the network (if
connections are short), or might change each day (if connections
can be long). This is necessary to mitigate correlation over
time.
o If a device can roam between networks, and more than a few bits of
entropy exist in the IPv6 interface identifier, then make sure
that the interface identifier can vary per network as the device
roams. This is necessary to mitigate location tracking.
5. IANA Considerations
This document has no actions for IANA.
6. Security Considerations
This entire document is about security considerations and how to
specify possible mitigations.
7. Informative References
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, DOI 10.17487/RFC3972, March 2005,
<http://www.rfc-editor.org/info/rfc3972>.
[RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD)
for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,
<http://www.rfc-editor.org/info/rfc4429>.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<http://www.rfc-editor.org/info/rfc4941>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<http://www.rfc-editor.org/info/rfc4944>.
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